The disclosures of the following priority applications are incorporated herein by reference in their entireties: Japanese Patent Application No. 11-107747, filed Apr. 15, 1999; Japanese Patent Application No. 11-284498, filed Oct. 5, 1999; and Japanese Patent Application No. 2000-35910, filed Feb. 14, 2000.
1. Field of Invention
This invention relates to a diffractive optical element and its fabrication method, an illumination device provided with the diffractive optical element, a projection exposure apparatus, and an exposure method. In particular, the invention relates to a device that illuminates a mask pattern for a semiconductor integrated circuit, a liquid crystal device, or the like, and an exposure method using the illumination device and a projection exposure apparatus that is suitable to the illumination device.
2. Description of Related Art
A process that is generally called photolithography is used for circuit pattern formation on a semiconductor substrate or the like. In this process, a reticle (mask) pattern is transferred onto a substrate such as a semiconductor wafer. First, a photosensitive photoresist is coated on the substrate, and a circuit pattern is transferred to the photoresist by an irradiated optical image, formed, e.g., from a transparent part of a reticle pattern. Furthermore, in a projection exposure apparatus, an image of a circuit pattern to be transferred, which was formed on the reticle, is projected and exposed onto the substrate (wafer) via a projection optical system. An illumination optical system of this projection exposure apparatus includes an optical integrator such as a fly eye lens to make an intensity distribution of the illumination light irradiated onto the reticle homogeneous. The following describes the reason why the intensity distribution of the illumination light irradiated onto the reticle is made homogeneous by using an optical integrator such as a fly eye lens.
FIG. 27A is a schematic diagram of an optical system of a projection exposure apparatus using a fly eye lens. The light beam generated from a light source (e.g., a KrF excimer laser) 100 is guided to a fly eye lens 103 via a beam expander optical system 101 and an oscillating mirror 102. Furthermore, the light emitted from the fly eye lens passes through an aperture diaphragm and illuminates a reticle 105 via a condenser lens 104. The pattern on the reticle 105 is then projected by a projection optical system 106 onto a substrate 107. The surface of the reticle 105 and the input surfaces of the respective lenses that constitute the fly eye lens 103 are located at conjugate positions relative to the condenser lens 104. Accordingly, the light beam input to the fly eye lens is divided by element lens units of the fly eye lens, and the divided light beams are then overlapped on the reticle surface. Because of this, even if there is a significant distribution of contrast difference in, for example, a Gaussian distribution of the light beam input to the fly eye lens, this distribution does not become significant at the element lens units of the fly eye lens, and is made to be uniform on the reticle surface because they overlap each other, and illumination distribution with extremely high homogeneousness can be obtained on the reticle surface 105.
A system is conventionally known in which processing such as beam splitting and overlapping thereof is repeated twice, and this system is hereafter called a double fly eye lens system. One example of an optical system of a conventional projection exposure apparatus using a double fly eye lens is shown in FIG. 27B. A shape of a light beam from a light source 201 such as an excimer laser is converted into a light beam with an arbitrary cross-sectional shape via an expander 202. The light beam then is input to a first fly eye lens (second light source) 205 formed of a plurality of optical elements via a mirror 203 and a quartz prism 204 for alleviating polarization of the light beam, and a plurality of second light source images are formed at the output surface of the first fly eye lens 205. The light beams output from the plurality of second light sources are condensed by a relay lens 206 and are superimposed on each other so as to homogeneously illuminate the input surface of a second fly eye lens 207. As a result, a number of third light source images that is equal to the product of the number of lens elements of the first fly eye lens and the number of lens elements of the second fly eye lens can be formed. Furthermore, the diameter of the light beam from the third light source is restricted by a diaphragm 208, condensed by condenser lens groups 209 and 211 (which includes bending mirror 212), and are superimposed so as to homogeneously illuminate a pattern on a reticle or mask 213. Here, a field diaphragm 210 to determine an illumination area is arranged among the condenser lens groups 209 and 211. Furthermore, based on the illumination light that has been homogeneously illuminated, a pattern formed on the reticle or the mask 213 is projected onto a substrate 215, which is the object of optical exposure, via projection lenses 214.
The characteristics of a system called a double fly eye lens as compared to a system using only one fly eye lens are described below. Furthermore, in order to simplify the description, the system using only one fly eye lens is called a single fly eye lens system.
(1) With respect to the effect that makes the illumination light that illuminates a reticle homogeneous, the greater the number of divisions of the fly eye lens (i.e., the more lens units within the fly eye lens), the more significant the effect becomes. However, the fabricating cost of the fly eye lens is substantially proportional to the number of divisions of the fly eye lens. Because of this, if many beam splittings are implemented by a single fly eye lens system, the fabricating cost of the lens becomes unacceptable. In the double fly eye lens system, the number of divisions of the first fly eye lens multiplied by the number of divisions of the second fly eye lens becomes the total number of divisions of the optical system. Accordingly, in a double fly eye lens system, there is an advantage that an illumination system with high performance can be obtained without unacceptably high fabricating cost. For example, if the first fly eye lens has 100 divisions and the second fly eye lens has 100 divisions, an illumination system that is equivalent to 10,000 (=100xc3x97100) divisions can be obtained at the fabricating cost of two lenses with 100 divisions.
(2) In a single fly eye lens system, the light distribution of the light source is input to the fly eye lens as-is. Therefore, if the light distribution changes with oscillation of the light source or the like, the spatial coherence of the projection exposure apparatus changes, which is not desirable. However, in the double fly eye lens system, the light distribution input to the second fly eye lens has been made homogeneous by the first fly eye lens. Accordingly, the light distribution hardly changes even if the light source is oscillated or the like. Therefore, there is an advantage such that it is difficult to affect the image performance even if oscillation or the like is generated in the light source.
(3) Another advantage of the double fly eye lens system is that the amount of change of the illumination homogeneousness when the aperture diaphragm is replaced, that is, the amount of change from an ideal Koehler illumination state, is less.
In addition to the above considerations, performance capability such as, e.g., resolution, which is demanded for these exposure apparatus has been approaching the theoretical limit. As is generally well known, a setting value of the optimum constant (e.g., numerical aperture of a projection lens, and numerical aperture of an illumination system, or the like) of the optical system varies depending on the pattern of a reticle. However, a device is demanded such that the optimum constant of the optical system can be selected according to the pattern of the mask because exposure is performed near the theoretical limit of the device performance capability.
Considering this fact in an illumination system, at least the numerical aperture of the illumination system needs to be variable. For the double fly eye lens system shown in FIG. 27B to be made with a variable numerical aperture, the diameter of the aperture diaphragm 208 could be made to be variable just like a diaphragm of a camera, or the diaphragm could be made to be switchable. However, if the diaphragm diameter is merely switched, in the case of changing the diaphragm diameter to a small diaphragm diameter, the area in which the light beam is shielded becomes large, and illumination power deteriorates.
Illumination power deterioration in this type of exposure apparatus means throughput deterioration. This increases the cost of the fabricated product. The profit margin per product is extremely low, especially for memory products, so the fabricating cost is a particularly important factor in the field of fabricating semiconductors or the like. Because of this, one of the most important issues in various specifications of exposure apparatus is xe2x80x9cillumination power,xe2x80x9d and it is necessary to avoid illumination power deterioration as much as possible.
In the double fly eye lens system, as a strategy against illumination power deterioration, a method has been proposed in which the first fly eye lens is switched to a lens having a different focal length along with a diaphragm. For example, in the case of making the aperture diaphragm diameter small, the first fly eye lens is switched to a lens having a long focal length. By this technique, because the light beam is condensed in the vicinity of the center of the second fly eye lens input surface, illumination power hardly deteriorates even if the aperture diaphragm diameter is small.
Thus, if the aperture diaphragm diameter merely changes, illumination power deterioration can be avoided by switching the focal length of the first fly eye lens. However, recently, there is a case in which a diaphragm having a shape other than a round shape is used as an aperture diaphragm. Examples are the ring diaphragm shown in FIG. 28 and the multiple aperture diaphragm shown in FIG. 29.
The aperture diaphragms of FIGS. 28 and 29 are briefly explained. When the pattern of the reticle becomes micro-small and exposure is performed near the resolution limit of the device, among the light beams generated from the aperture diaphragm of the illumination system, it is only the light generated from the part surrounding the aperture diaphragm that contributes to resolution; the light generated from the center of the aperture merely decreases the image contrast. In other words, when the information of the reticle is transmitted to the wafer, it is only the light generated from the surrounding part of the aperture diaphragm that provides the information transmission energy. The light generated from the center of the aperture merely generates so-called noise. Therefore, it is preferable that light should not be generated from the center of the aperture diaphragm. The diaphragm shown in FIG. 28 was employed to address this phenomenon. The diaphragm shown in FIG. 29 is a diaphragm that is used when the pattern to be resolved is limited to only lines of the vertical and horizontal directions. In this case, the light generated from upper, lower, right and left portions of the aperture diaphragm merely generates noise. The diaphragm shown in FIG. 29, which further shields the top, bottom, right and left of the aperture diaphragm, was employed to address this phenomenon.
In the case of this type of aperture that is not a round aperture, there is a problem in that loss of the light amount can occur because only one part of the transparent light amount of the fly eye lens is used; i.e., the light is shielded by the variable aperture diaphragm in many parts including the center, i.e., in the vicinity of the optical axis. Illumination power deteriorates at the reticle surface, and throughput decreases.
This invention addresses the above-mentioned and/or other problems.
An object of this invention is to provide a diffractive optical element that effectively converts an input light beam to a light beam having a predetermined cross-sectional shape, and to provide a method of fabricating such a diffractive optical element.
Other objects of this invention are to provide an illumination device that is provided with the diffractive optical element and that can form various light intensity distributions at a specified surface, to provide a projection exposure apparatus that is suitable to use with the illumination device, and to provide an exposure method that uses the projection exposure apparatus.
In order to address the above and/or other problems, one aspect of this invention provides a diffractive optical element that converts an input light beam to an output light beam having a specified cross-sectional shape, in which the diffractive optical element includes a plurality of partial optical elements. Each of the plurality of partial optical elements converts the input light beam to respective specified partial light beams. Although each of the partial light beams does not have the specified shape, a sum of the partial light beams matches the specified cross-sectional shape of the output light beam.
Preferably the diffractive optical element includes a plurality of basic optical elements, each including a plurality of the partial optical elements.
Furthermore, it is preferable that the partial optical elements have a shape corresponding to a phase distribution that combines a phase distribution of a rotationally symmetrical lens component and a phase distribution of a diffractive grid that deflects input light in a specified direction.
Another aspect of this invention provides a method of fabricating a diffractive optical element having at least one basic optical element that includes a plurality of partial optical elements, the diffractive optical element converting an input light beam into an output light beam having a specified cross-sectional shape. The method includes the steps of:
dividing the specified cross-sectional shape into a plurality of partial areas; and
arranging the plurality of partial optical elements, which correspond to the plurality of partial areas, into a condensed state, thereby defining the basic optical element.
Furthermore, another aspect of this invention provides an illumination device that illuminates a mask in which a specified pattern is formed, including a light source, a light beam converter, an optical integrator and a condenser optical system. The light source supplies a light beam. The light beam converter includes a diffractive optical element as described above, and receives the light beam from the light source. The optical integrator receives the output light beam from the light beam converter and forms a substantially planar light source at a specified plane based on the light beam that is diffracted by the light beam converter. The condenser optical system guides the light beam from the optical integrator to the mask. The light beam converter can change an optical intensity distribution on the specified plane.
Additionally, another aspect of this invention provides a projection exposure apparatus including: a first stage for holding the mask, the illumination device set forth above that illuminates the mask, a second stage for holding a substrate to be exposed, and a projection optical system to project and expose an image of a pattern of the mask that has been illuminated by the illumination device onto the substrate.
Another aspect of this invention provides an exposure method, including the steps of (1) illuminating a mask utilizing the illumination device set forth above, and (2) forming an image of a pattern of the illuminated mask onto a substrate that is coated by a photosensitive material.
Another aspect of this invention relates to an optical homogenizer having a plurality of basic optical elements formed by etching a substrate, each of the basic optical elements illuminating different areas which are overlapped but shifted relative to each other to average the noise in a specified plane.
An optical homogenizer is not limited to a diffractive element, but includes an optical intensity homogeneous element having a refractive type element, both a diffractive element and a refractive element, or the like.
It is preferable that an amount by which an area to be illuminated by each of the basic optical elements is shifted is equal to an amount that satisfies a relationship that fills in concave and convex parts of an intensity distribution of noise pattern due to Fresnel diffraction, or a noise pattern due to fabricating errors. Additionally, while the substrate can be a glass substrate, it is preferable to use a fluorite substrate if the wavelength of the input light beams becomes short.
By this technique, after the light beam passes the basic optical elements, the interference noise generated at the input surface of the optical integrator can be made homogeneous. As a result, illumination homogeneousness at the output surface of the optical integrator improves, and the illumination homogeneousness on the reticle plane (and on the wafer plane) is ultimately improved.
Another aspect of this invention relates to an optical homogenizer having a plurality of basic optical elements formed by etching a substrate, in which the basic optical elements are arranged so as to be shifted relative to each other based on an intensity cycle of an interference noise pattern generated by the plurality of basic optical elements.
By this technique, the interference noise formed by the optical integrator can be effectively made homogenous. As a result, much higher illumination homogeneousness can be obtained.
In conventional devices, a first fly eye lens was arranged at a position of the optical homogenizer, and the alignment accuracy of the element lens (that is, the basic optical elements) of the first fly eye lens was determined by the outer-diameter difference of the element lens. Because of this, a random average effect was obtained. On the contrary, the alignment accuracy of the basic optical elements is much higher in the etched optical homogenizer of this aspect of the invention. Therefore, a patterned average, that can obtain a better result than a random result, can be implemented. Furthermore, this effect is also good for a refractive type optical homogenizer that is fabricated by etching, instead of a diffractive type element.
Another aspect of this invention relates to a method of making an optical homogenizer comprising the steps of:
creating a reticle on which a pattern of a basic optical element is formed;
coating a sensitive material onto a substrate;
reduction exposing the pattern onto the sensitive material on the substrate via a reduction projection optical system;
shifting a position of the substrate and repeating the reduction exposing step, thereby generating latent images of the basic optical element in an arrayed state on the sensitive material; and
developing and etching the latent images.
It is also preferable that the reticle includes a plurality of patterns of the basic optical element.
By this technique, compared to a proximity method, pattern resolution can be improved. As a result, patterning with less fabricating errors is possible, and transmission efficiency and illumination homogeneousness of the optical homogenizer are further improved. Additionally, because this is not a batch exposure method, an optical homogenizer with a better interference noise decreasing effect can be fabricated.
Furthermore, because a pattern of a plurality of basic optical elements is written on a reticle in advance, it is possible to decrease the number of exposures that are needed for fabricating the optical homogenizer. Accordingly, fabrication throughput of the optical homogenizer is improved, and the cost of the optical homogenizer can be decreased. Furthermore, writing errors generated during the plurality of basic pattern writings vary, and the respective elements are eventually used in parallel, so there is an effect in that the reticle writing errors are canceled. That is, illumination homogeneousness of the optical homogenizer is further improved.
It also is preferable that the pattern is formed on the reticle in the creating step by writing the pattern on the reticle with an electron beam.
By this process, because the optical homogenizer is fabricated by electron beam writing, patterning can be performed with high accuracy. As a result, patterning with less fabricating errors is possible, and the transmission efficiency and illumination homogeneousness of the optical homogenizer are further improved.
It is further preferable that the pattern is formed on the reticle in the creating step by projection exposing the reticle with the pattern from an original substrate that was fabricated by electron beam writing.
By this process, patterning can be performed with high accuracy. As a result, patterning with less fabricating errors is possible, and the transmission efficiency and illumination homogeneousness of the optical homogenizer are further improved. Furthermore, if a reticle is fabricated by exposing a substrate that has been EB written a plurality of times, the final number of exposures that are needed for fabricating the optical homogenizer can be decreased. That is, fabrication throughput of the optical homogenizer is improved, and the cost of the optical homogenizer can be decreased.